The present application is based on and claims priority from Japanese Patent Applications: 2000-191737 filed Jun. 26, 2000; 2000-213090, filed Jul. 13, 2000 and 2001-148258, filed May 17, 2001 , the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a voltage regulator of a vehicle AC generator.
2. Description of the Related Art
In an AC generator, a switch is connected in series with a field coil to turn on if a voltage of a phase-winding becomes higher than a predetermined value. In this generator, field current is supplied to the field coil when this generator starts generation. However, if a leak current flows in the armature coil due to short-circuiting, a noise voltage is caused by the leak current. If this noise voltage is detected as a generation-start signal, field current is erroneously supplied to the field coil.
JP-A-6-276796 proposes a voltage regulator that solves the above stated problem. The voltage regulator has a generation detection circuit that has a terminal connected to a phase-winding of an AC generator and a bypass resistor that is connected between the terminal and a ground. The bypass resistor bypasses most of the leak current that flows into the armature coil.
In order to prevent erroneous detection of the generation start signal, it is important to lower the resistance of the bypass resistor. However, when the output power is generated and output current flows through the bypass resistor, the bypass resistor consumes a considerable electric power. This lowers the efficiency of the AC generator and heats the portions of the voltage regulator surrounding the bypass resistor.
Each of JP-A-3-215200, JP-A-6-284598 and PCT International Publication 8-503308 discloses a signal detection circuit that detects and amplifies a voltage difference between two phase-coils. This detection circuit can correctly detect the generation start signal even if leak current flows into the armature coil.
However, such a signal detection circuit necessitates complicated wiring arrangement in the AC generator, thereby increasing parts and man-hour.
In addition, such a detection circuit that detects residual magnetic flux needs a rectifier unit for rectifying a self-excited AC voltage and a comparator for comparing the rectified voltage and a reference voltage. Because such self-excited voltage to be detected when an engine is started is about 0.4 V, it is very difficult to rectify such low AC voltage and to compare it with a reference voltage accurately.
Therefore, a main object of the invention is to provide a simple voltage regulator of a vehicle AC generator that can detect such self-excited voltage accurately.
A voltage regulator according to a feature of the present invention, comprises first means for detecting the voltage level or frequency of a self-excited voltage induced in a phase-winding, second means for supplying field current to a field coil when the self-excited voltage is detected, and
third means including a bypass circuit connected to a ground, for reducing resistance of the bypass circuit when the second means does not supply the field current and increasing the resistance of said bypass circuit when the second means supplies field current to the field coil.
Even if leak current flows into the armature coil while detecting the self-excited voltage, the leak current can be eliminated by bypassing it through the bypass circuit, so that the self-excited voltage can be detected accurately. After the self-excited voltage is detected, the resistance of the bypass circuit is increased to thereby decrease power consumption by the bypass circuit.
A voltage regulator according to another feature of the invention comprises a switching circuit for controlling field current to be supplied to the field coil, a switch control circuit for controlling the switching circuit according to a self-excited voltage induced in a phase-winding, a power circuit connected to the switch control circuit and a power-circuit drive circuit including a pulse conversion circuit for converting the self-excited voltage into a binary pulse signal. The power-drive circuit drives the power circuit for a period controlled by the pulse signal.
If a peak voltage of the self-excited voltage becomes a threshold level for the binary pulse signal, the power circuit supplies the field coil of the AC generator with field current that corresponds to the binary pulse signal. The duty ratio of the pulse signal and the field current increase as the rotation speed of the generator increases. Therefore, the AC generator is prevented from abruptly generating the output power. It is not necessary to provide a rectifier unit for rectifying the excited voltage.